1. Field of Invention
The present invention relates generally to an electric vehicle (EV), hybrid EV (HEV) or fuel cell EV (FCEV), and specifically to a hierarchical method and system for an EV, HEV, or FCEV to simulate the negative powertrain torque (deceleration force) of a traditional internal combustion engine vehicle when the accelerator pedal is released and only the electric motor is providing torque to the powertrain.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption by and pollutants from automobiles and other vehicles powered by an internal combustion engine (ICE) is well known. Vehicles powered by electric motors attempt to address these needs. However, electric vehicles have limited range and limited power capabilities and need substantial time to recharge their batteries. An alternative solution is to combine both an ICE and electric traction motor into one vehicle. Such vehicles are typically called hybrid electric vehicles (HEV""s). See generally, U.S. Pat. No. 5,343,970 (Severinsky).
The HEV has been described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) is a vehicle with an engine (most typically an ICE) that powers a generator. The generator, in turn, provides electricity for a battery and an electric traction motor coupled to the drive wheels of the vehicle. There is no mechanical connection between the engine and the drive wheels. Further, a parallel hybrid electrical vehicle (PHEV) is a vehicle with an engine (most typically an ICE), battery, and electric traction motor that combine to provide torque to the drive wheels of the vehicle.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both the PHEV and the SHEV. The PSHEV is also known as a torque (or power) split powertrain configuration. Here, the engine torque can be used to power a generator and/or contribute to the necessary wheel or output shaft torque. Further, the PSHEV reduces emissions and fuel consumption under certain conditions by turning the engine off. The PSHEV can be used to generate electricity to the battery or can contribute to the necessary wheel or output shaft torque. The traction motor is used to contribute to the necessary wheel or output shaft torque and can also be used to recover braking energy to the battery if a regenerative braking system is used.
The desirability of combining the ICE with an electric motor is clear. The ICE""s fuel consumption and pollutants are reduced with no appreciable loss of performance or range of the vehicle. Nevertheless, there remains substantial room for development of ways to optimize HEV operation.
One such area of development is the improvement of the overall HEV drive-ability and feel consistent with traditional ICE vehicles. This is especially true when the HEV""s engine is not contributing torque to the vehicle""s powertrain or is not even running. When the electric motor is solely providing powertrain torque, there is very little or no drag on the powertrain after the driver releases a speed control such as an accelerator pedal. Typically, the driver of a traditional ICE vehicle expects a coast-down force in vehicle speed when the accelerator pedal is released because of the effect of engine braking.
Engine braking is well known and is typically characterized by two types of negative powertrain torque including engine friction and pumping (compression) losses. Engine friction loss occurs during engine braking because the engine, although unfueled, is still connected to the powertrain. Engine pumping loss refers to the compression of air in each engine cylinder as it moves through its stroke. Engine braking is expected by the driver and allows reduction of vehicle speed without applying force to the brake pedal.
Various ways for an electric vehicle to simulate the ICE""s negative torque when the accelerator is released are known in the prior art. Gardner, et. al., U.S. Pat. No. 4,103,211 (1978) describes a system of providing a motor armature regenerative current path through a load resistor by maintaining motor field current when the speed control is released. The resistance element serves to limit the magnitude of the current produced by the armature and provide a power sink or load to absorb the regenerative energy. Thus, the combination of a function generator, a diode, and a resistance element acts to provide the expected negative powertrain torque. The Gardner invention also discloses that the system could be adapted for bidirectional motion (i.e., forward and reverse) by, for example, switches for reversing the motor field winding connections.
Regenerative braking (regen) causes vehicle coast-down by capturing the kinetic energy of the vehicle. In conventional vehicles, kinetic energy is usually dissipated as heat at the vehicle""s brakes or engine during deceleration. Regen converts the captured kinetic energy, through a generator, into electrical energy in the form of a stored charge in the vehicle""s battery. Consequently, regen also reduces fuel usage and emission production.
In Nicholls, et. al., U.S. Pat. No. 4,691,148 (1987), a control circuit for electric vehicles is described that includes applying regen during coasting and braking. The regen is switched on by releasing the accelerator or slightly engaging the brake pedal. Mechanical brakes are added when additional brake pedal force is applied. During this regenerative braking, the circuit between the battery pack and the motor must remain connected even though the accelerator pedal is released. Normally, the battery pack would be disconnected from the motor. The control circuit is calibrated to reduce speed gradually using regen when brake pedal braking is not desired.
Carter, et. al., U.S. Pat. No. 5,578,911 (1996, Chrysler Corporation), describes a continuously variable regenerative control for an electric vehicle. The control provides the ability to tailor the regenerative (or braking) effect of the electric motor to match that of the internal combustion engine. This drivability characteristic is familiar to and desired by many vehicle operators.
Although the desire and need for drivability feel when the accelerator is released is known, to some extent, for a solely electric vehicle, there is a need to develop a more sophisticated method and system for a consistent coast-down feel including all electric powertrain applications.
Accordingly, it is an object of this invention to provide a method and system for an EV, FCEV or HEV to provide a negative torque to a powertrain with an electric motor, comprising: an operator movable accelerator through a range of distance from a rest position to a full power position; a means to transmit when the accelerator is released to a vehicle system controller (VSC) ; a means to transmit to the VSC when the electric motor is a sole source of torque to the powertrain; a control module within the VSC to determine that both the accelerator is released and the electric motor is solely providing torque to the powertrain; and a coast-down strategy that receives the output of the control module and commands negative torque to the powertrain when the control module has determined the accelerator is released and the electric motor is the sole torque provider to the powertrain.
Another object of the invention provides negative powertrain torque in a manner calibrated in amount and duration to the amount of vehicle deceleration expected or desired by the driver of a traditional internal combustion engine (ICE) vehicle when the accelerator pedal is released.
It is a further object of this invention to maximize energy recovery using regenerative braking (regen) in a vehicle with an electric motor powertrain while maintaining a consistent and expected deceleration.
Deceleration can occur using a variety of negative powertrain torque strategies including the following: dissipating the kinetic energy of the vehicle as heat in the electric motor (xe2x80x9cpluggingxe2x80x9d); adding a resistive load to the powertrain, whereby deceleration occurs from controlling the motor as a generator and generating electrical power to store in a battery; and activating mechanical brakes, whereby the vehicle""s kinetic energy is dissipated as heat in brake rotors and drums.
It is a further object of this invention to provide a method and system that will select the appropriate method of energy dissipation based on vehicle operating conditions such as battery state of charge, engine/motor temperature, resistive load temperature, such that deceleration is it consistent regardless of operating conditions. The system configuration may contain some or all of strategies. It should also be understood that additional methods could be utilized as an obvious extension of this logic.